Neutrons: The Ship Effect & Soil Moisture Measurement
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Neutrons: The Ship Effect & Soil Moisture Measurement

By Dr. Zoomie

Good morning, Dr. Zoomie, I’ve got a question about neutrons for you. Actually, a couple of things about neutrons, if you don’t mind. It started when I was measuring neutron radiation from my soil moisture gauge and I got some readings that were sort of higher than I expected. I asked the RSO at the state university nearby and he asked if I was near a big bridge or building. When I told him I was he said something about “ship effect” and told me my readings would be normal if I moved away from it. And he was right! But then I was wondering, how did he know I near the bridge, and what’s that got to do with ships? And for that matter, what are they using the neutrons for anyhow to measure soil moisture? Come to think of it, where do the neutrons come from? Sorry to have so many questions – like the paper says, enquiring minds want to know. Thanks, Dr. Zoomie!

Good questions, Grasshopper! And I hope you don’t mind if I answer your questions out of order – I think it’ll make more sense that way.

Let’s start with the “where do the neutrons come from” part. In a soil moisture gauge, they usually come from a radioactive source that mixes americium-241 (which gives off alpha radiation) and beryllium (a light metal); using the chemical symbols for americium and beryllium, these are called AmBe (pronounced “ambee) sources. A different way of producing neutrons is with a radionuclide that spontaneously fissions, emitting neutrons with each fission. Californium (specifically Cf-252) is a good spontaneous fission neutron source. Anyhow, when an alpha particle strikes a beryllium atom it causes the beryllium atom to, among other things, give off a neutron; that’s the source of the neutrons your gauge is producing.

As to why neutrons are there in the first place, when a neutron strikes an atom that’s close to the same mass as the neutron, it’s likely to bounce back (to be reflected) rather than to be absorbed. Water, with two hydrogen atoms (remember, hydrogen is simply a lone proton, which has nearly the same mass as a neutron) reflects neutrons fairly well. So if you have a neutron source shooting neutrons into the soil and a neutron detector (thoughtfully installed in your soil gauge) that’s counting the neutrons that are reflected back, you’ll count more neutrons when there’s more water in the soil – when the moisture content is higher. Incidentally, neutron sources can be used to measure the concentration of anything with a lot of hydrogen, including, say, asphalt (a hydrocarbon compound). So the same neutron sources can be used to measure the amount of asphalt in the paving compound being put down on a road, or to measure the amount of tar in the compound used to coat rooftops – both places these gauges are used.

So that addresses your two easy questions – and now, on to the more difficult one!

I had a similar question several years ago – I was doing neutron surveys in a big city and noticed that I was getting unexpectedly high count rates in some locations. So I called a physicist at a local laboratory who did a lot of work in neutron detection. In his case, he was helping people look for nuclear weapons (which emit low levels of neutrons) and the ship effect could skew their results. It would be a shame to – literally – think there was a nuclear weapon under every bridge or in every container ship. In any event, here’s a nutshell version of what he told me.

To start, any time you turn on a neutron detector you should get some counts – if you don’t get a small count rate (the actual number will depend on the type of neutron detector you have) then it means your neutron detector might not be working properly. Those neutrons are formed in the upper atmosphere; they form when cosmic rays slam into atoms in the atmosphere. Sometimes the neutrons are absorbed by the atom, sometimes they knock a proton out of the nucleus, sometimes they knock a few neutrons out of the nucleus, and sometimes they cause the nucleus to break into two or more parts. Some of the neutrons that are formed will reach the ground, some of any of these particles will go on to cause further reactions with other atoms in the atmosphere that will produce still more neutrons. And some of these neutrons will eventually cause a count or two in your neutron detector. This is where the background neutron counts come from. And it’s sort-of where the ship effect comes from.

If you’re standing in a field then the cosmic ray neutrons reach the ground (and your neutron detector) unimpeded. But if you’re on a boat next to a ship (or if you’re standing beneath a bridge or next to a large building), some of those neutrons will strike the ship, the bridge, or the building, and when they do, they knock out protons, neutrons, or cause the atoms to spall the same as when atoms are struck higher in the atmosphere. But when it happens so close to where your instrument is sitting it’s possible for multiple secondary particles to strike your detector from a single neutron that hits the ship (or bridge or building) first, creating a spray of particles. So instead of intercepting a single neutron, your detector is hit by several. What you see if a cluster of counts every time this happens and your instrument, averaging all of the counts together over a period of time, shows a higher count rate than if you were standing in the middle of a field. This is the ship effect (

Interestingly, you can see something similar in airplanes at high altitudes; when a cosmic ray strikes the skin of the airplane it can cause the same sort of reactions, and if the cosmic ray is a heavy atomic nucleus (the nucleus of an iron atom, for example), the incoming cosmic ray can break up into several parts, including neutrons.

Ultimately, the ship effect is interesting – and it’s something you can see yourself if you’re making neutron measurements. In general, the larger your neutron detector is, the more counts you’ll see. But, when all is said and done, it’s not going to affect your health (or mine) in the slightest. So there’s that! Ultimately, when you get down to it, the ship effect will affect your instrument readings, it can confuse things a bit until you remember you read about it once, and it’ll have absolutely no effect on your health…unless you tend to get worked up whenever your instruments give a little click.